OK, I thought you were talking about giant (size not type) slaying weapons and vulnerabilities and resistances based on size. Climbing on another creature was least interesting, IMO, of what I thought
@Ruin Explorer was talking about. I mean even the 2014 DMG has guidelines for climbing on another creature and I personally have never felt like I needed anything other than the standard ability/skill check system myself for such activities.
Though, now I think about our (my groups) standard houserules for HP, BHP, & AC cover the basics of size difference pretty well actually.